I should note that I haven't been playing a lot of Tau in this edition, but when I have, I've been taking between 4-8 markerlights, mostly on Pathfinders and Cadre Fireblades.
Basically, markerlights are okay if you get 1 hit and great if you get 5+ hits. Everything in-between is fairly situational and not really worth it unless you pack a LOT of seeker missiles.
Essentially, then, you're really only trying to get 1 hit on the units you plan on eliminate that turn. You don't need a hell of a lot of markerlights for that and, since the benefits they give are fairly incidental (re-rolling 1s on Tau BS just isn't that amazing), they aren't such a huge target priority as they were in editions past. In the rare cases you need to get a lot of hits on a single target, that's what the Uplinked Markerlight strategem is for.
So, for a 2000 point game, I rather doubt you'll need more than 10, but I wouldn't take fewer than 6, just because you're going to want a fair amount of coverage.